The teen moves to Cheltenham on-loan to the end of the season.
"Ethan is a very talented player and he comes here now to add that bit of quality in the final third," Cheltenham Town manager Michael Flynn told the club's website.
"There was a lot of interest in him, so I'm really glad that we got it over the line.
"He's a player that makes the keeper work when he gets into the final third. I'm excited to be working with him."
Williams also said: “I’m really proud. I really enjoyed my first day here and now I can’t wait to start playing games and get minutes under my belt.
“I always look to offer goals, assists and hard work. That fits the Club’s repertoire as the manager has alluded to me so hopefully I can do that.
“Personally I’m really looking forward to the experience.